Taylor S R, Preiser H, Sandow A
J Gen Physiol. 1969 Sep;54(3):352-68. doi: 10.1085/jgp.54.3.352.
I(-), CH(3)SO(4) (-), and ClO(4) (-), like other previously studied type A twitch potentiators (Br(-), NO(3) (-), SCN(-), and caffeine), lower the mechanical threshold in K depolarization contractures of frog skeletal muscle. In potentiated twitches, I(-), Br(-), CH(3)SO(4) (-), ClO(4), and SCN, as already reported for NO(3) (-) and caffeine, slightly shorten the latent period (L) and considerably increase the rate of tension development (dP/dt) during the first few milliseconds of the contraction period. Divalent cations (8 mM Ca(2+), 0.5-1.0 mM Zn(2+) and Cd(2+)) raise the mechanical threshold of contractures, and correspondingly affect the twitch by depressing the tension output, increasing L, and decreasing the early dP/dt, thus acting oppositely to the type A potentiators. These various results form a broad, consistent pattern indicating that electromechanical coupling in the twitch is conditioned by a mechanical threshold as it is in the contracture, and suggesting that the lower the threshold, in reference to the raised threshold under the action of the divalent cations, the more effective is a given action potential in activating the twitch as regards especially both its early rate and peak magnitude of tension development. The results suggest that the direct action by which the various agents affect the level of the mechanical threshold involves effects on E-C coupling processes of the T tubular and/or the sarcoplasmic reticulum which control the release of Ca for activating contraction.
碘离子（I⁻）、甲基硫酸根离子（CH₃SO₄⁻）和高氯酸根离子（ClO₄⁻），与其他先前研究过的A型抽搐增强剂（溴离子（Br⁻）、硝酸根离子（NO₃⁻）、硫氰酸根离子（SCN⁻）和咖啡因）一样，可降低青蛙骨骼肌钾去极化挛缩中的机械阈值。在增强的抽搐中，碘离子、溴离子、甲基硫酸根离子、高氯酸根离子和硫氰酸根离子，正如先前已报道的硝酸根离子和咖啡因那样，会略微缩短潜伏期（L），并在收缩期的最初几毫秒内显著提高张力发展速率（dP/dt）。二价阳离子（8 mM钙离子（Ca²⁺）、0.5 - 1.0 mM锌离子（Zn²⁺）和镉离子（Cd²⁺））会提高挛缩的机械阈值，并相应地通过降低张力输出、增加L和降低早期dP/dt来影响抽搐，其作用与A型增强剂相反。这些各种结果形成了一个广泛且一致的模式，表明抽搐中的机电耦合如同挛缩中一样，受机械阈值的制约，并且表明，相对于二价阳离子作用下升高的阈值而言，阈值越低，给定动作电位在激活抽搐方面就越有效，尤其在其早期速率和张力发展峰值幅度方面。结果表明，各种试剂影响机械阈值水平的直接作用涉及对横管和/或肌浆网的电 - 机械偶联过程的影响，这些过程控制着用于激活收缩的钙的释放。
